A conventional NMR detection system uses a magnet, a probe and a spectrometer. The magnet polarizes the sample by producing a strong magnetic field in a sample region. Superconducting magnets produce the strongest magnetic fields, and operate at low temperatures requiring a Dewar and cryogenic equipment. The probe is positioned within the magnet and supports the sample and the RF coils or resonators generating and detecting the RF magnetic fields used to stimulate and detect the resonance signals from the nuclear spins. Probes with cryogenic resonators also contain a Dewar system and a temperature control system that maintains a very low temperature environment for these resonators and maintain a near room temperature environment for the sample. Cryogenic resonators comprise cold normal metal resonators or high temperature superconducting (HTS) resonators that operate at temperatures below 100 K and typically in the range of 20 to 30 K. The resonator comprises an inductive section usually in the form of a coil and a capacitive section that may be in the form of distributed capacity or a separate capacitor. The NMR spectrometer comprises electronic circuits with a transmitter to generate RF energy that is applied to a transmitter resonator, a RF receiver used to amplify and detect any NMR signals induced into a receiver resonator, recording, and display circuits for handling, storing and displaying the NMR data, and a controller that can be programmed to carry out the desired experiment. The controller can also be used to control accessories that may be added, such as sample changers and a RF switch power source.
NMR is a powerful technique for analyzing molecular structure. The technology is also used in magnetic resonance imaging (MRI) to study physical structures and blood flow patterns. It is also an insensitive technique, compared to other techniques, for structure determination. To gain maximum sensitivity, NMR magnets and spectrometers are designed to operate at high magnetic field strengths, employ low noise preamplifiers and RF probes with cryogenic resonators that operate at cryogenic temperatures. The cryogenic resonators use cold normal metal transmitter/receiver coils or preferably transmitter/receiver coils made with high temperature superconducting (HTS) materials operating at low temperatures. The transmitter/receiver coils are the probe coils that stimulate the nuclei and detect the NMR response from the sample, and therefore are placed very close to the sample to provide high sensitivity. The cryogenically cooled normal metal coils and the HTS coils have the highest quality factor, Q, and yield the best sensitivity.
In a typical experiment one or more RF transmit pulses are applied to the probe resonator to excite a selective nuclear resonance signal. This is followed by a reception period where the transmitter is silent and the receiver is activated to detect and record any response signal produced by the nuclei. In some systems the same coil or resonator is used to produce the transmit RF magnetic field and to receive the response signal of the nuclei. In other systems separate resonators are used for transmitter and receiver. In some imaging systems a single transmitter RF coil or resonator is used for exciting the nuclei, and one or more receiver coils or resonators are used to detect the response. In these systems the transmitter coil or resonator be operational at cryogenic temperatures or at near room temperatures. The receiver resonators are normally arranged to provide little or preferably no coupling with the transmitter resonator. This is done to prevent the receiver resonator from distorting the RF field produced by the transmitter resonator. Any small residual coupling between the transmitter and receiver resonators allows for a RF voltage to be induced in the windings of the receiver coil or resonator which in turn causes a circulating current to flow through the receiver coil windings producing a RF field in the sample region thereby distorting the RF field produced by the transmitter resonator. Residual coupling between the transmitter and receiver coils reduces the sensitivity during the receive phase since the small NMR signal currents in the receiver coil windings induce currents in the transmitter coil windings causing a loss in sensitivity. Direct coupling of the RF fields produced by the precessing nuclei also induce currents in the transmitter coil causing a loss in sensitivity. In systems with multiple receiver coils it may also be desirable to minimize the coupling between separate receiver coils.
In systems that use the same coil or resonator for transmit and receive another problem can occur. After a transmit pulse, the energy remaining in the transmitter coil decays with a time constant τ=Q/ω, where ω is the RF frequency in radians/second and Q is the quality factor of the transmitter coil. This energy causes a distortion of the NMR signal if the receiver is activated before this transmitter energy has sufficiently decayed.
In NMR probes with cold normal metal coils or superconducting transmitter and receiver coils these problems can be particularly severe as the coil Q-values in these probes are particularly high thereby increasing the time constant τ and the effect of any coupling between coils. In simple resonator systems Q can be defined as the ratio of inductive (or capacitive) reactance to the coil resistance. Since superconducting coils have very low resistance at RF frequencies, the Q-values are high. By reducing the Q the effect of this mutual coupling is reduced, however it is desirable to do this without reducing the sensitivity of the probe that is gained by the high Q-values. By causing a small section of a superconducting coil to become non-superconducting either by increasing the current through the section above its critical current or by increasing its temperature of the section above the superconducting critical temperature, TC, introduces more resistance into that section thereby reducing the Q of the coil. Even before the current exceeds the critical current, the HTS material becomes non-linear showing an increase in inductance and a small increase in resistance. In some cases these smaller changes may be adequate to achieve a desired reduction of coil Q or detuning of the coil.
To gain this advantage of high Q-values and reduce the detrimental effects, it is desirable to reduce the Q of the transmitter resonator while the receiver resonator maintains a high Q-value during the receive phase. Similarly it is desirable to reduce the Q of a receiver resonator while the transmitter is applying a RF field to the nuclei. Any RF currents induced in the receiver resonator by the transmitter pulse will produce an additional RF field in the sample region that distorts the RF field produced by the transmitter resonator.
In many cases the direct coupling of the transmitter resonator to the receiver resonator cannot be sufficiently eliminated to avoid the above problem. U.S. Pat. No. 4,763,076 teaches to use switching diodes to selectively connect and disconnect portions of an RF resonant circuit in response to a DC control signal to change the Q-value and frequency of a resonant circuit. The DC control signal selectively forward biases and reverse biases the switching diodes. The use of switching diodes appears practical in normal metal RF resonators operating at room temperature, but do not appear practical in systems using cryogenic resonators. Placing diodes in these circuits greatly lowers their quality factor Q thereby largely eliminating the advantage of cryogenic resonators.
U.S. Pat. No. 6,727,702 B2 describes various methods to “de-Q”, i.e. reduce the Q-value of HTS coils used in MRI by heating a short section of the superconducting trace to a temperature above its critical temperature Tc. While heated above its Tc, the superconductor loses its superconductivity, and the electrical resistance of that section greatly increases thereby lowering the quality factor Q of the coil. An additional method is disclosed that relies on heat generated by an RF transmit pulse to switch a superconductor in a circuit out of a superconducting state. If a superconducting resonator receives enough RF energy from a transmit pulse, the resultant electrical current will exceed the superconducting critical current in one or more points in the circuit heating the material above the critical temperature making it much more resistive. This increase in resistance will limit the RF power absorbed by the inductor. The switching on-to-off and off-to-on times occur on a fast enough to limit absorption from the transmit pulse, while recovering in time to receive the RF signal. The circuit may comprise one or more of these switches with points with reduced critical current formed by a narrowing of the line-width of a section of the superconducting coil or by damage of the superconducting material within a restricted area forming a so called superconducting RF switch.
The technique of using the current induced in a receiver coil by the transmitter pulse to reduce the Q of the receiver coil might be applicable in a few situations, however normally the current induced in the receiver coil by a transmit pulse remains below the critical current in the coil even if the receiver circuit has a superconducting RF switch. The technique is not practical in the inverse situation, i.e. to reduce the Q of the transmitter coil by currents induced in the receiver coil. The currents induced in the transmitter coil either directly or indirectly by the nuclear signals are far too small produce a current sufficiently strong to cause a section of the transmitter coil or RF switch to exceed the critical current and become a normal conductor. Also the technique is not applicable to reducing the decay time τ, after a transmit pulse.